1. Field of the Invention
The invention relates generally to the class of multimode optical fibers and in particular to a new class wherein the core of the fiber has flat surfaces rather than being round and to their use with diode lasers in medical applications.
2. Information Disclosure Statement
Medical laser devices, of the types used for medical treatments, generally employ a laser, capable of generating a significant amount of optical power, coupled to a fiber optic delivery device for delivering the optical power from the laser source to a treatment site for cauterization or other use.
Present technology offers devices with powerful glass rod, dye or Argon:ion lasers as the source of optical power coupled with a delivery system consisting essentially of a connector, one or more flexible fibers and a distal tip, suitably configured for the intended treatments. A typical optical fiber has a circular core coated with a cladding having an index of refraction lower than that of the fiber core, to redirect light rays attempting to escape the optical fiber core back into the core. The difference in refractive index can be accomplished, for example, by doping the cladding with fluorine or by doping the fiber core with germanium.
Semiconductor laser sources are convenient because of their relatively low cost, and their ability to generate optical outputs of significant power. Present technology offers semiconductor laser sources capable of generating as much as a watt or more of optical power. This output occurs over a generally rectangular area of, for example, about 200 by 10 .mu.m with divergences of 40 and 20 degrees in the respective axis. Effective treatment requires that the available output power be delivered to the treatment site with a certain minimum optical power density (watts/cm.sup.2). Conventional optical fibers have a circular cross section. To capture as much optical output power as possible, it is conventional to use an optical fiber having a core diameter substantially equal to the major dimension of a semiconductor laser source (200 .mu.m in the example above). However, the cross sectional area of a 200-.mu.m diameter optical fiber core is about 3.15.times.10.sup.4 .mu.m.sup.2, whereas the cross sectional area of the example 200 by 10 .mu.m laser output is about 2.0.times.10.sup.3 .mu.m.sup.2. Therefore, the energy density of the rectangularly shaped radiation input to the fiber is transformed by multiple internal reflections within the circular optical fiber core to a generally uniform energy density across the entire output cross section of the optical fiber core. Thus, the input energy density of a one-watt diode laser at the input end of the fiber core (one watt/(2.times.10.sup.-5)cm.sup.2 =50 kW/cm.sup.2) is reduced by about a factor of 16 (plus coupling and attenuation losses) to a value less than that required to effect for most medical treatments.
This reduction in output energy density is a main reason why current technology does not use semiconductor laser sources. Increasing the power of the laser source is not feasible for low-cost semiconductor lasers. This has lead to the use of higher-cost laser sources such as, for example, Nd:YAG, Dye, Holmium:YAG, Ar:Ion or other. These lasers, besides being capable of higher output power, have typically circular symmetric emission characteristics, and thus can be easily focused onto the circular input end of a fiber core.
The present state-of-the-art lasers are, however, relatively expensive, complex and maintenance-intensive. These drawbacks limit their use--and, indeed, the whole scope of present-day medical laser practice--essentially to large clinics. With the current availability of laser diodes, new areas can be made accessible, since the laser diodes are small, lightweight and easily configurable into essentially maintenance-free, potentially cheap laser systems, provided that the energy density required for effective medical treatment can be delivered to the treatment site.
Optical methods, including lenses, are only partially effective in correcting the shape mismatch between laser diode output and conventional optical fiber input. At best, a lens is able to equalize the divergences in the two dimensions and to compress the rectangular diode laser output by a ratio of about 2:1 or 3:1, rather than the 10:1 or 30:1 requires for effective coupling of power density to the optical fiber.
Power density, however, is a critical value in medical laser treatments, as it controls the physical effects of the radiation on the tissue. For instance vaporization of tissue can only be achieved above a certain power density threshold.
The above mentioned problems clearly hinder the penetration of laser diodes into the market.